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Low-cost LED dimming circuitry

An electronic circuitry for dimming of an LED light source is described. In particular applications of LEDs, such as in a wake-up light providing a gradually increasing light intensity to gently wake up a user, a high dimming range of the LEDs is required. In a wake-up light, for example, the required dimming range is over 1,000,000:1 to ensure that the initial step of the wake-up light cycle is unnoticeable to the user even in a dark environment. When using a PWM controller for controlling the average current through the LEDs, the PWM controller must have a 21-bits resolution in order to achieve this dimming range. In that case the PWM controller only varies the pulse duration of the current pulses, but not the amplitude of the current pulses. This high resolution is however only required for minimizing the initial step of the wake-up light cycle. The resolution required for the main portion of the wake-up light cycle is much lower.

A solution is described to realize such a high dimming range with the use of a low-cost PWM controller that has a much lower resolution, e.g. 16 bits. In particular the ID proposes an electronic circuitry that combines PWM dimming and analog dimming, i.e. dimming by a reduction of the amplitude of the current pulses. In particular the analog dimming is realized by applying current pulses that have a slowly rising current value, i.e. current pulses that have a substantial ramp-up time required to reach the maximum current value. As a result, when the pulse duration is shorter than the ramp-up time required to reach the maximum current value, the pulse does not reach the maximum current value. For such short pulse durations, the average current value of the PWM pulses is much lower than the average current value of rectangular PWM pulses having a similar pulse duration and maximum current value, so that a larger dimming range can be achieved.

In the following figure, a number of PWM current pulses is shown, which have a constant ramp-up time (TRU) and a maximum current value (IMAX). The period time of the pulses is...